The present invention relates to stereoselective Ziegler-Natta catalyst compositions for use in the polymerization of propylene having improved control over polymerization activity and reactor process continuity through the use of carefully chosen mixtures of selectivity control agents. Ziegler-Natta propylene polymerization catalyst compositions are well known in the art. Typically, these compositions include a transition metal compound, especially a mixed titanium, magnesium and halide containing compound in combination with an internal electron donor (referred to as a procatalyst); a co-catalyst, usually an organoaluminum compound; and a selectivity control agent (SCA). Examples of such Ziegler-Natta catalyst compositions are shown in: U.S. Pat. Nos. 4,107,413; 4,115,319; 4,220,554; 4,294,721; 4,330,649; 4,439,540; 4,442,276; 4,460,701; 4,472,521; 4,540,679; 4,547,476; 4,548,915; 4,562,173; 4,728,705; 4,816,433; 4,829,037; 4,927,797; 4,990,479; 5,028,671; 5,034,361; 5,066,737; 5,066,738; 5,077,357; 5,082,907; 5,106,806; 5,146,028; 5,151,399; 5,153,158; 5,229,342; 5,247,031; 5,247,032 and 5,432,244.
Catalyst compositions designed primarily for the polymerization of propylene or mixtures of propylene and ethylene generally include a selectivity control agent in order to affect polymer properties, especially tacticity or stereoregularity of the polymer backbone. As one indication of the level of tacticity, especially the isotacticity of polypropylene, the quantity of such polymer that is soluble in xylene or similar liquid that is a non-solvent for the tactic polymer is often used. The quantity of polymer that is soluble in xylene is referred to as xylene solubles content or XS. In addition to tacticity control, molecular weight distribution (MWD), melt flow (MF), and other properties of the resulting polymer are affected by use of a SCA as well. It has also been observed that the activity of the catalyst composition as a function of temperature may be affected by the choice of SCA. Often however, a SCA which gives desirable control over one polymer property, is ineffective or detrimental with respect to additional properties or features. Conversely, an SCA that is effective in combination with one procatalyst may not be effective when used in combination with a different procatalyst.
With regard to the temperature dependence of catalyst activity, it is known that the use of certain aromatic carboxylic acid esters, especially ethyl p-ethoxybenzoate (PEEB), in combination with a Ziegler-Natta procatalyst composition containing an ester of an aromatic monocarboxylic acid, exemplified by ethyl benzoate, results in an inherently self-extinguishing catalyst composition with respect to temperature. That is, such compositions are less active at moderately elevated polymerization temperatures, especially temperatures from about 80 to 130° C. Using such compositions, less reactor fouling or sheeting is observed, and run-away reactors due to increased polymerization rates at elevated temperatures, are largely eliminated. Disadvantageously, such catalyst compositions employing aromatic carboxylic acid esters, exemplified by PEEB, tend to possess lower overall polymerization activity and result in the production of polymers having relatively low isotacticities and increased oligomer contents, all of which are generally undesired results. Interestingly, the combination of PEEB with a procatalyst containing a dialkyl ester of an aromatic dicarboxylic acid, such as diisobutylphthalate (DIBP) as an internal electron donor generally results in poor polymerization activity and production of polypropylene polymers having low isotacticity (high XS).
In contrast, alkoxysilane SCA's, exemplified by dicyclopentyldimethoxysilane (DCPDMS), methylcyclohexyldimethoxysilane (MCbDMS) and n-propyltrimethoxysilane (NPTMS) generally form isotactic polymers having improved physical properties, when used in combination with an dialkyl ester of an aromatic dicarboxylic acid, such as DIBP, as an internal electron donor. Disadvantageously however, these catalyst compositions are not generally self-extinguishing, thereby resulting in polymerization process control problems, especially sheeting and formation of large polymer chunks due to hard to control temperature excursions allowing polymer particles to form agglomerates. For example, the polymerization activity of a typical catalyst composition containing DIBP as internal electron donor with DCPDMS as SCA generally increases as polymerization temperatures rise, especially at temperatures from 67 to 100° C.
Use of mixtures of SCA's in order to adjust polymer properties is known. Examples of prior art disclosures of catalyst compositions making use of mixed SCA's, especially mixtures of silane or alkoxysilane SCA's include: U.S. Pat. Nos. 5,100,981, 5,192,732, 5,414,063, 5,432,244, 5,652,303, 5,844,046, 5,849,654, 5,869,418, 6,066,702, 6,087,459, 6,096,844, 6,111,039, 6,127,303, 6,133,385, 6,147,024, 6,184,328, 6,303,698, 6,337,377, WO 95/21203, WO 99/20663, and WO 99/58585. References generally showing mixtures of silanes with monocarboxylic acid ester internal electron donors or other SCA's include: U.S. Pat. Nos. 5,432,244, 5,414,063, JP61/203,105, and EP-A-490,451.
Despite the advances occasioned by the foregoing disclosures, there remains a need in the art to provide an aromatic dicarboxylic acid ester internal electron donor containing Ziegler-Natta catalyst composition for the polymerization of olefins, wherein the catalyst composition retains the advantages of alkoxysilane SCA containing catalyst compositions with regard to polymer properties but additionally possesses improved temperature/activity properties. Especially desired are such compositions that are inherently self-extinguishing with regard to catalyst activity as a function of temperature, thereby leading to reduced polymer agglomerate formation and improved polymerization process control.